Using Fuji lenses for astrophotography

Started 3 months ago | Discussions thread
TOMMMMMM Junior Member • Posts: 46
Re: Using Fuji lenses for astrophotography

getochkn wrote:

35mm f2, single shot. 10s exposure, f2, iso 1600 or maybe 3200

Montanawildlives wrote:

I know that third-party lenses like the Samyang 12mm f/2 are very popular for astrophotography on Fuji cameras, but I'm wondering what Fuji branded lenses (I'll allow Zeiss as well) people are using for astrophotography. Seems like they all have some limitations (e.g., 10-24 is only f/4, 16mm f/1.4 has substantial CA until stopped down, etc.), but I'm wondering anyway.

Someone in the astro forum was saying that the 27mm f2.8 was actually better in some way than a wider lens with a wider aperture (e.g., the Samyang 12mm f2) for some reason I really didn't understand...that the REAL thing to look at is diameter in relation to the aperture, so the 27mm, even though it is only 2.8, would allow more light (or more light per something or other). Didn't understand a bit of it.


Nice photo! But that doesn't really answer the question...

I think the reasoning is that a longer focal length at a similar f stop as a wide angle will have a larger area open to receiving light.

F-stop = focal length/aperture diameter. As the focal length increases, the aperture diameter must also increase to keep the f-stop the same. Since the area for aperture diameter is calculated as (pi*diameter_aperture^2)/4, you can see that a focal length twice as long lets in 4 times the amount of light (aperture area is 4 times larger).

The problem with using long focal lengths in astro photography though is that the star movements will blur quicker at longer focal lengths than wide focal lengths. So while you may be letting in more light, you will have blurry, messy looking stars. Generally, the rule for "sharp" non blurry stars is exposure time = 500/focal length. Luckily here, there is no exponential function, so doubling focal length only halves the amount of exposure time for sharp stars (see Star trackers to avoid this).

So back to your original question: the 27mm f2.8 will allow approximately 2.6x the amount if light in over the 12mm f2.

On the other hand, the 12mm f2 can expose for 2.25xas long as the 27mm f2.8 (for similar star sharpness), so it's a trade-off you need to consider. If you aren't too worried about blurry stars, then the 27mm f2.8 will give you a brighter image in a shorter exposure, but generally blur controls. Either way, both lenses should offer comparable results, however the optics of the 12mm may be better for astro due to it's low coma. Also, the 12mm is manual focus which is much better suited for astrophotography, over the fly-by-wire focusing 27mm f2.8.

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